Background

A G3 (Strong) geomagnetic storm peaked at Kp=7 overnight on 3 to 4 April 2026, the strongest during a crewed deep-space transit since the Apollo programme. Four Artemis II astronauts were coasting beyond Earth's magnetosphere in unshielded translunar space when the storm struck. NASA published zero CREW radiation dose data through the entire event.

The storm was the peak of an escalation chain that began with an X-class solar flare on launch day (31 March), progressed through a G1 watch, a G2 storm with a coronal mass ejection forecast, and an M7.5 flare causing an R2 radio blackout. Six HERA radiation sensors and personal dosimeters aboard Orion collected readings continuously, but no numbers reached the public.

By Day 5 the storm had fully resolved. NOAA forecast maximum Kp of 3.67, well below the G1 threshold. A NASA Q&A confirmed the CREW would use approximately 5% of their lifetime radiation caps across the full ten-day mission, the only quantification offered. What fraction accumulated during the Kp=7 peak remains undisclosed.

Common Questions

Is the Artemis II crew safe from the solar storm?

Six radiation sensors collected data continuously and NASA confirmed the CREW would use about 5% of their lifetime caps, but no specific storm exposure numbers were published.Source: background

What is a G3 geomagnetic storm?

A Strong-level storm on the 1-5 NOAA scale, measured at Kp=7 on a 0-9 index. It can disrupt satellites and pose radiation risks to astronauts outside Earth's magnetosphere.Source: background

Why did NASA not publish radiation dose data during Artemis II?

NASA treated CREW dose data as information not shared during flight. The entire G3 storm passed without a single number reaching the public despite six sensors collecting continuously.Source: background

What is the Kp index?

A 0-9 scale measuring geomagnetic disturbance. Below 4 is quiet, 5+ is a minor storm (G1), 7 is strong (G3), and 9 is extreme (G5).Source: quick_facts